In this document the invention is explained by way of the application example of wet-cleaning of a gas turbine compressor. The invention may however also be used in other fields of technology, such as e.g. in power station technology and other fields, wherever a gas flow is interspersed with a liquid.
All gas turbines suffer from contamination of the compressor blades. This phenomenon is caused by solid and liquid particles in the intake air which despite filtration of the intake air enter the turbine installation and remain adhered to the compressor blades. Such particles may comprise dust, pollen, insects, oil, sea salt, industrial chemicals, uncombusted hydrocarbons, soot particles, etc. The contamination of the compressor blades leads to losses in the efficiency and power of the whole installation of up to approx. 10% and more. In order to avoid or reduce these losses, one attempts to clean the compressor blades. From the state of the art, various methods and devices for cleaning compressor blades are known.
Traditional cleaning methods are based on soft abrasion by way of rice, nutshells or likewise during operation of the installation. These soft abrasion agents are admixed into the intake air and combusted in the turbine. These simple methods however are not suitable for modern turbines, particularly for those whose compressor blades are provided with protective coatings and whose combustion chamber as well as turbine blade cooling systems are provided with the most modern technology.
For cleaning modern gas turbine compressors three methods are used today:    (i) The manual cleaning method with the installation at a standstill. This method results in an efficient part-cleaning, but in practise may only be carried out within a planned standstill, on inspection or overhaul of the installation. Without opening the compressor cylinder only the first stator row may be cleaned manually, i.e. by hand.    (ii) The off line wet-cleaning method (i.e. cleaning with a starter motor, crank wash, at standstill and with a cooled-down turbine). As a cleaning liquid one uses water, mixtures of water or solvent based compressor cleaners, or such mixtures with an anti-freeze addition. This method is efficient since the complete compressor blading of the rotor as well as the stator and not merely the first stator row may be cleaned. It however has the disadvantage that it needs to be carried out with the turbine at standstill and thus causes losses in production.    (iii) The on line wet-cleaning method (i.e. wet cleaning during operation) with the cleaning liquids as stated under (ii). With this method the compressor blades surface are wetted uniformly and as complete as possible with the cleaning liquid and the dirt particles are released. This cleaning method may be carried out with the gas turbine operating so that no production losses are caused.
The present invention relates to the on line cleaning method (iii).
As of today's state of the art and with the injection nozzles used for on line cleaning, up to now one may differentiate between low-pressure nozzles and high-pressure nozzles. The former operate at a pressure of approx. 3 to 15 bar and produce droplets with diameters of approx. 30 to 1000 μm whilst the latter operate at a pressure of approx. 15 to 90 bar and produces droplet diameters of approx. 3 to 30 μm. Usually one strives for a fine atomisation of the cleaning liquid so that a wetting of the compressor blades as uniform and as surface-covering as possible is performed, in order to ensure their thorough cleaning. The atomisation may also cause a temperature depression of the ingested compressor air by evaporation of the injected liquid mass. Despite this side effect which is desirable per se one strives for as low as possible a mass flow of cleaning liquid in order to avoid or at least reduce further side effects in the compressor (possible erosion) and in the turbine (possible flame-outs) produced by the mass flow of the cleaning liquid including the entrained dirt particles.
According to the common teaching of the state of the art an efficient wetting of the compressor blades is achieved by uniformly distributed droplets. The droplets must be so small that they do not erode the compressor blades and so light so that they are not deflected too much downwards due to the force of gravity and do reach the compressor blades. The design of the injection nozzles is crucial in order to meet the mentioned requirements and thus to ensure an efficient cleaning. This is because the air speed in the air intake duct due to the narrowing of the cross sections is accelerated up to approx. 180 m/s at the entry of the first compressor row. In order to achieve a good droplet distribution in the air flow it is of advantage, according to the power output class of the engine, to provide a larger number (up to approx. 40 and more nozzles) of injection nozzles arranged in the compressor air intake duct.
A method and a device for wet-cleaning a compressor are known from U.S. Pat. No. 5,193,976 (S. Kolev et al.). According to this document a cleaning liquid is injected into the compressor air intake duct by means of one or several injection nozzles. The atomised spray is produced in the form of a cone whose cone angle is approx. 90°. The injection nozzles are atomisation nozzles which are located in an adjustable ball joint and mounted in the compressor air inlet duct wall. This method and these injection nozzles are very suitable for small and medium sized gas turbines of approx. 5 up to 180 MW outputs. Larger gas turbines however have outputs of 180 up to 350 MW and more and thus require correspondingly larger air intake cross sections as well as correspondingly longer compressor blades in particular for the first stator row. For such large high-power output gas turbines the injection nozzles disclosed in U.S. Pat. No. 5,193,976 are no longer performing adequately in order to achieve an efficient and uniform interspersion of the air cross section at the injection plane. The liquid droplets atomised at the nozzles are caught by the intake air flow much too soon, and are deflected from their original trajectory path and carried away. By way of this an efficient interspersion of the whole intake air flow with the liquid droplets becomes very difficult.
From the state of the art there are known nozzles from which simultaneously a liquid and a gas, usually air may be discharged. The discharged air with these so-called two-substance nozzles mostly serves for breaking up or atomising the liquid jet into very small droplets. The publication WO-98/01705 discloses a two-substance nozzle for atomising a liquid. The nozzle is manufactured by way of micro-structuring-layered semiconductor materials. It serves to produce as equal as possible liquid droplets with a small diameter of 10 μm or less. U.S. Pat. No. 6,267,301 (J. Haruch) teaches an enhanced or double atomisation of the liquid. Air is admixed to the liquid in the nozzle pre-chamber in order to achieve a higher discharge velocity and a more efficient atomisation. Furthermore air is discharged at an angle of incidence onto the liquid jet exiting from a liquid discharge orifice, i.e. the air has at least a speed component perpendicular to the liquid jet. By way of this one achieves a fine atomisation as is required for humidification and cooling purposes.
The document EP-0'248'539 discloses a nozzle for atomising a liquid fuel and its admixture with air in a so-called premix burner. In one embodiment form the fuel is discharged from a liquid injection orifice into a first pre-combustion chamber and from there it is discharged into a second pre-combustion chamber. In the second pre-combustion chamber the fuel is swirled with air from a first annular swirl body. The mixture is discharged together with air from a second annular swirl body into the combustion chamber.
There are further known two-substance nozzles having one or more layers of air enveloping or enclosing a liquid jet. Thus e.g. according to U.S. Pat. No. 2,646,314 (D. J Peeps) or U.S. Pat. No. 4,961,536 (J. Y. Correard) an annular layer of air is aligned coaxially and parallel to the liquid jet. U.S. Pat. No. 5,452,856 (J. Pritchard) discloses a nozzle with which a discharged liquid spray may be modified with respect to size and shape by way of simultaneously discharged air. Such nozzles for example are used in spray pistols for atomising varnishes and paints. They are however not suitable for the application for wet-cleaning a large gas turbine compressor where the speed of the intake air at the injection location is approx. 30-80 m/s and up to approx. 180 m/s before the first stator row. These nozzles were designed for the injection of a liquid in standard, atmospheric surrounding conditions. The extremely finely atomised droplets may either not penetrate the boundary layer or are immediately deflected by the airflow resulting in a very poor interspersion of the airflow and thus a poor wetting of the blade surfaces. A large portion of the liquid would thus be pressed by the flow onto the walls of the air intake duct. This portion of the liquid may not be used for the cleaning and may cause erosion problems, mainly on the first compressor rotor blade row.
U.S. Pat. No. 5,738,281 (Z. Zurecki et al.) discloses a gas nozzle with which the discharged gas is shielded from the surroundings by way of an auxiliary gas discharged simultaneously. The auxiliary gas is discharged through a porous medium in a manner such that it forms a cushion enclosing the gas.
To conclude, it may be said that on the one hand the two-substance nozzles known from the state of the art are designed for very different applications and thus are not suitable for interspersing strong, high velocity gas flows with liquid droplets. The known nozzles designed for wet-cleaning gas turbine compressors on the other hand are only wetting adequately the blade surfaces and thus achieving good cleaning results with gas turbines of small and medium power output classes.